Measurement of transmitted and reflected millimeter wave and microwave signals is useful for characterizing the properties of a device under test. Such properties, typically measured by a vector network analyzer, include the complex (i.e., magnitude and phase) reflection and transmission coefficients. Devices under test are, for example, high frequency circuit components, equipment that performs material characterization or nondestructive testing and evaluation, imaging systems, high-frequency transceivers, and ranging systems. Currently, taking measurements of transmitted and/or reflected signals at high frequencies involves expensive vector network analyzers, which employ phased-locked input sources and heterodyne detection processes to isolate or determine the frequencies to be measured. In addition to being prohibitively expensive, current vector network analyzers are considered bulky for applications where handheld, customized, and/or special purpose measurement modules are needed.
Accurate vector measurement of high frequency signals (i.e., their magnitude and phase) reflected back from and transmitted through a device under test (“DUT”) helps characterize the electrical performance of the DUT. By way of analogy, FIG. 1 demonstrates these signals as a wave directed towards a slab 100, wherein the slab reflects and/or transmits an incident wave 102. FIG. 1 diagrammatically illustrates the incident wave 102 striking the slab, a reflected wave 104, and a transmitted wave 106. Based on measurements of the reflected and/or transmitted waves, the DUT can be characterized using known methods to determine its properties, such as its electrical impedance or the amount of signal distortion it causes to the supplied signal.
Among measurement devices known in the prior art for the measurement of the phase and magnitude of high frequency signals, FIGS. 2A-2D illustrate their operation. As shown in FIG. 2A, a vector network analyzer (VNA), generally indicated as 198, operably connects a stimulus source 214 (e.g., a built-in phased-locked oscillator) to a DUT 202. The source 214 supplies an incident signal 204 to the DUT 202. In a conventional test set-up, a plurality of directional coupling devices 210 separate the incident signal 204 from a reflected signal 206 and a transmitted signal 208. In turn, a receiver/detector 200 collects the signals 204, 206, 208 via the multiple directional coupling devices 210. In a typical vector network analyzer, such as VNA 198, the detector/receiver 200 comprises a tuned receiver, such as a super heterodyne receiver, allowing a high frequency input signal to be translated to a lower frequency through a process known in the art as down-conversion. The VNA 198 uses a reference signal (e.g., the incident signal 204 or a signal supplied from a second high-frequency source phase-locked to the primary stimulus source 214) in the down-converting process to mix with other signals. To down-mix the signals 204, 206 in this manner, VNA 198 must maintain signal separation and prevent the signals from interacting to form a combined or standing wave.
Other exemplary prior art techniques for measuring the phase and magnitude of high frequency signals are illustrated in FIGS. 2B-2D. As shown, these additional methods do not utilize heterodyne/tuned receivers. FIG. 2B illustrates a slotted-line method of measuring a complex reflection coefficient of a standing wave. This method utilizes a single detector probe 220 to gather measurements on a transmission line 222. However, the probe 220 must be physically moved along the length of the transmission line 222 to obtain the multiple measurements needed to accurately measure the complex reflection coefficient. Since the distance 224 between the DUT (not shown) and the detector 220 is a factor in the complex coefficient calculation, each change in position must be accurately measured and recorded contemporaneously with the corresponding transmission line measurement. This repeated repositioning and distance measuring makes automation burdensome, adding to the cost and complexity of implementing the slotted line method shown in FIG. 2B.
Referring now to FIG. 2C, a sampled line method requires at least three fixed detector probes 240 along a length of transmission line 242 to obtain the desired measurements unambiguously. Although this prior art method avoids the repositioning of a single detector probe (see FIG. 2B), probe interaction and non-identical probes introduce measurement errors that significantly affect measurement accuracy.
Another method known in the art for measuring incident and reflected signals is the so-called perturbation two-port (“PTP”) method illustrated in FIG. 2D. The PTP method is based on using a combination of two-port perturbation (PTP) networks 260 inserted before the DUT 262 and a scalar network analyzer 264 (i.e., high quality reflectometer realized with a directional coupler). Multiple PTP networks 266, each representing different electrical characteristics, are required. Each PTP network changes/transforms the sought DUT reflection coefficient appearing at the input of the scalar network analyzer 264. When these multiple PTP networks are used along with a multi-step calibration routine, the sought after reflection coefficient can be determined (both magnitude and phase) after measuring the magnitude of the reflection coefficient seen at the input of the scalar network analyzer. Like the prior art VNA described above with respect to FIG. 2A, the PTP method demands signal separation. In the PTP method embodied in FIG. 2D, directional couplers (within the implementation of the scalar network analyzer) are again required to maintain separation between the incident and reflected signals. Furthermore, the PTP method requires that some of the used PTP networks have losses associated with them. Without signal separation and lossy PTP networks, the PTP method fails to work.
The conventional VNA 198 in FIG. 2A maintains signal separation of the incident, reflected, and transmitted signals, and employs one or more specialized tuned, heterodyne receivers to selectively isolate and analyze the signals collected, all of which increase the size, cost, and complexity of the device. Other prior art devices described above perform measurements without heterodyne procedures, but due to limitations inherent in their designs, measurement inaccuracies significantly undermine the useful operation of these devices. As noted above, it remained for the present inventors to discover a method and system of measuring the transmitted and reflected signals of a device under test without the cost and complexity of a tuned receiver, requiring directional coupling devices, or the introduction of significant inaccuracies in the measurement process.